Dopamine wiki:”
Dopamine plays a major role in the brain system that is responsible for reward-driven learning. Every type of reward that has been studied increases the level of dopamine transmission in the brain, and a variety of highly addictive drugs, including stimulants such as cocaine and methamphetamine, act directly on the dopamine system.[2] There is evidence that people with extraverted (reward-seeking) personality types tend to show higher levels of dopamine activity than people with introverted personalities. Several important diseases of the nervous system are associated with dysfunctions of the dopamine system. Parkinson’s disease, an age-related degenerative condition causing tremor and motor impairment, is caused by loss of dopamine-secreting neurons in the substantia nigra. Schizophrenia has been shown to involve elevated levels of dopamine activity in the mesolimbic pathway and decreased levels of dopamine in the prefrontal cortex.[3][4] Attention deficit hyperactivity disorder (ADHD) is also believed to be associated with decreased dopamine activity.[5]”

“As a result, the normal dopamine-triggered firing from such neurons, in essence, launches two different types of neuronal ammunition, causing “cosignaling.”””

“Also, they wrote that their findings may explain why treatment of children with fluoxetine can induce depressive symptoms in adulthood. The researchers wrote that, since serotonin plays a vital role in neuronal development, disruption by fluoxetine of the normal serotonin levels during development could be responsible for such behavioral abnormalities.”